Welcome to healthcare magic
You seem to be having a condition called as chronic urticaria
In majority of the cases, it is not possible to find out a cause for the urticaria, which is when we call it is “idiopathic”.
In few of the cases, it could be due to a specific food, medications and infections, or auto-immune.
As you have Hashimoto's chance of your urticaria being auto-immune is higher.
Some times treatment of thyroid
status cures urticaria, but urticaria should not decide the adjustment of thyroxine
. Your TSH levels and T3, T4 levels should decide your thyroxine dose. Your endocrinologist
will take a call on that.
for long duration is not harmful. Continue taking antihistamine regularly in a minimum dose required to control wheals. It might stop one day, till then be comfortable.
Hope this helped